Drive trains of hybrid vehicles comprise several drive units, that is, a first drive unit formed as an internal combustion engine and a second drive unit formed as an electric motor, a transmission, and an output. With such a drive train of a hybrid vehicle, the first drive unit acts on a first transmission shaft of the transmission, whereas the second drive unit acts on a second, different transmission shaft of the transmission. The transmission of the drive unit comprises several shift elements, which are typically designed as frictional-locking shift elements, such as (for example) clutches or brakes. At the output of the drive train, an output torque that depends on the torque required by the driver is provided. In defined operating situations of such a drive train, it is necessary to open a previously locked shift element of the transmission. If a frictional-locking shift element is to be opened, this may occur in a simple manner by the frictional-locking shift element being brought into slip. This gives rise to friction torques or drag torques, which have negative effects on consumption. Therefore, in transmissions of drive trains, frictional-locking shift elements are increasingly being replaced by positive-locking shift elements, such as claw shift elements. However, so that they can be opened, such positive-locking shift elements must be transferred into a load-free state or positioned in a load-free manner. However, with drive trains known from practice, this is possible only to an insufficient degree.
DE 10 2012 007 622 A1 discloses a method for carrying out a shifting in a transmission of a drive train, with which a first drive unit formed as an internal combustion engine drives a generator, which is coupled through a power electronics unit to an engine transmission unit. The engine transmission unit comprises a second drive unit formed as an electric motor and a transmission, whereas the transmission is designed as a planetary transmission.
There is a need for a method of operating a drive train with at least two drive units engaging at different transmission shafts, that is, a method by means of which one shift element of the transmission can be positioned in a load-free manner, and in a manner in which, upon the load-free positioning of the shift element to be opened, the output torque prevailing at the output deviates as little as possible from the torque required by the driver.